1. Field of the Invention
The present invention relates to a plasma processing apparatus for processing an object to be processed such as a semiconductor wafer by use of a plasma, more specifically, for plasma etching.
2. Description of the Related Art
In a semiconductor manufacturing process, a plasma has been conventionally employed for etching the surface of a semiconductor wafer (hereinafter referred to as "wafer"). In the plasma etching, ion energy is greatly influenced by a self bias electric potential (V.sub.DC) generated on a wafer. Hence, it is quite important to measure V.sub.DC in the semiconductor manufacturing process.
V.sub.DC is measured, for example, by a V.sub.DC measuring terminal provided to a clamp when a wafer is mechanically held by the clamp. However, such a V.sub.DC measuring terminal cannot be directly used when a wafer is electrostatically held by an electrostatic chuck formed of a conductive body coated with an insulating material such as a polyimide resin, since the conductive body is not exposed to a plasma. Hitherto, V.sub.DC is measured through monitoring by an appropriate lead wire, an end of which is connected to a high-frequency power supply passage for supplying a high-frequency power from the outside to the lower electrode of a processing chamber, and the other end of which is connected to a V.sub.DC monitor via an RF (radio frequency) filter.
In this case, however, if V.sub.DC is directly obtained by monitoring from the high-frequency power supply passage, the V.sub.DC value obtained is not accurate for two reasons. First, the outer peripheral surface of the lower electrode is usually insulated with an anodized aluminum. Second, an insulated electrostatic chuck is provided to the upper surface of the lower electrode, as mentioned above.
Jpn. Pat. Appln. KOKAI Publication No. 6-232088 discloses a technique for monitoring the V.sub.DC value by using a monitor terminal which is insulated from the power supply passage and independently provided around a substrate. However, since an independent monitor signal line must be provided in the lower electrode, the apparatus is complicated in structure. Accordingly, a simple plasma processing apparatus designed so as to obtain an accurate V.sub.DC value is desired.